Among automobile structural members used as skeletal frame members of an automobile vehicle body (body shell) having a monocoque structure, cited as the member on which a three-point bending load is supposed to act at a time of collision accident are a side sill, a bumper reinforcement, a center pillar, and so on.
A high three-point bending load is required of the above automobile structural member (hereinafter, referred simply to as an “automobile structural member) in order to secure a vehicle interior space at the time of collision or in order to transmit an impact load at the time of collision to another automobile structural member efficiently. Further, the automobile structural member itself is required to have an excellent impact absorption performance to three-point bending. In the present specification, the tree-point bending load and the impact absorption performance to the three-point bending which the automobile structural member has are combinedly referred to as a bending-crushing performance. Further, not only the bending-crushing performance but also weight reduction and cross section reduction for the purpose of fuel consumption improvement of an automobile is strongly required of the automobile structural member.
The automobile structural member is assembled, in general, by spot welding a first composing member made of a steel sheet molded into a hat shape in cross section having an outward flange and a closing plate being a second composing member made of a steel sheet, using the outward flange as a welding margin. In the present specification, the member with a hat-shaped cross section which has the outward flange is referred to as a hat member. In other words, the hat member has a hat-shaped transverse section shape made by two outward flanges, two bent portions each connected to the two outward flanges, two vertical wall portions each connected to the two bent portions, two edge line portions each connected to the two vertical wall portions, and one groove bottom portion to which the two edge line portions are connected.
Heretofore, various inventions have been suggested for the purpose of suppressing increase of a weight of a first composing member being a hat member and for obtaining an excellent bending-crushing performance.
Patent Literature 1 discloses an automobile structural member in which a reference range of (R+7) mm is set, with a radius of a bent portion being R, on at least one surface of two surfaces forming the bent portion from a center of the bent portion in a transverse section of a first composing member being a hat member molded by press bending of a steel sheet containing C: 0.05 to 0.3% (in the present specification, “%” related to a chemical composition means “mass %” as long as not mentioned otherwise) and Mn: 0.5 to 3.0%, one or a plurality of quench-strengthened portion(s) is (are) formed along the bent portion in the reference range by laser irradiation or high-frequency heating, and an occupancy ratio of 20% or more in relation to a total reference range length of a total width of the quench-strengthened portion in the reference range is secured for all the surfaces constituting all the bent portions of the first composing member. It is disclosed that according to this automobile structural member an impact crushing resistance can be improved effectively by small formation of the quench-strengthened portion while good press moldability is secured.
Patent Literature 2 discloses an automobile structural member having a barrel unit which has a closed transverse section shape formed by a first composing member being a hat member and a second composing member and in which the first composing member and the second composing member are spot-welded by an outward flange, the automobile structural member being interposed by filling a resin layer in a region sandwiched by a bent portion neighboring the outward flange and the second composing member. It is disclosed that according to this automobile structural member light weight and excellent bending-crushing performance are had.
On the other hand, Patent Literature 3 discloses an automobile structural member made by joining, by laser-welding, inward flanges of two composing members with recessed cross sections to each other, in a manner that bottom surface inner walls thereof face each other. It is disclosed that according to such an automobile structural member changing of a bending load property can increase an energy absorption amount.